kbuild: Do not pick up ARCH_{CPP,A,C}FLAGS from the environment

Initialize the ARCH_* overrides before including the arch Makefile, to
avoid picking up the values from the environment. The variables can
still be overriden on the make command line, but this won't happen
by accident.

Signed-off-by: Michal Marek <mmarek@suse.com>
diff --git a/Makefile b/Makefile
index 13270c0..dc868bc 100644
--- a/Makefile
+++ b/Makefile
@@ -597,6 +597,11 @@
 # Defaults to vmlinux, but the arch makefile usually adds further targets
 all: vmlinux
 
+# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
+# values of the respective KBUILD_* variables
+ARCH_CPPFLAGS :=
+ARCH_AFLAGS :=
+ARCH_CFLAGS :=
 include arch/$(SRCARCH)/Makefile
 
 KBUILD_CFLAGS	+= $(call cc-option,-fno-delete-null-pointer-checks,)